rust/compiler/rustc_codegen_llvm/src/va_arg.rs

use rustc_abi::{Align, BackendRepr, Endian, HasDataLayout, Primitive, Size};
use rustc_codegen_ssa::MemFlags;
use rustc_codegen_ssa::common::IntPredicate;
use rustc_codegen_ssa::mir::operand::OperandRef;
use rustc_codegen_ssa::traits::{
    BaseTypeCodegenMethods, BuilderMethods, ConstCodegenMethods, LayoutTypeCodegenMethods,
};
use rustc_middle::bug;
use rustc_middle::ty::Ty;
use rustc_middle::ty::layout::{HasTyCtxt, LayoutOf, TyAndLayout};
use rustc_target::spec::{Arch, Env, LlvmAbi, RustcAbi};

use crate::builder::Builder;
use crate::llvm::Value;
use crate::type_of::LayoutLlvmExt;

fn round_up_to_alignment<'ll>(
    bx: &mut Builder<'_, 'll, '_>,
    mut value: &'ll Value,
    align: Align,
) -> &'ll Value {
    value = bx.add(value, bx.cx().const_i32(align.bytes() as i32 - 1));
    return bx.and(value, bx.cx().const_i32(-(align.bytes() as i32)));
}

fn round_pointer_up_to_alignment<'ll>(
    bx: &mut Builder<'_, 'll, '_>,
    addr: &'ll Value,
    align: Align,
) -> &'ll Value {
    let ptr = bx.inbounds_ptradd(addr, bx.const_i32(align.bytes() as i32 - 1));
    let pointer_width = bx.tcx().sess.target.pointer_width;
    let mask = align.bytes().wrapping_neg() & (u64::MAX >> (64 - pointer_width));
    bx.call_intrinsic(
        "llvm.ptrmask",
        &[bx.type_ptr(), bx.type_isize()],
        &[ptr, bx.const_usize(mask)],
    )
}

fn emit_direct_ptr_va_arg<'ll, 'tcx>(
    bx: &mut Builder<'_, 'll, 'tcx>,
    list: OperandRef<'tcx, &'ll Value>,
    size: Size,
    align: Align,
    slot_size: Align,
    allow_higher_align: bool,
    force_right_adjust: bool,
) -> (&'ll Value, Align) {
    let va_list_ty = bx.type_ptr();
    let va_list_addr = list.immediate();

    let ptr_align_abi = bx.tcx().data_layout.pointer_align().abi;
    let ptr = bx.load(va_list_ty, va_list_addr, ptr_align_abi);

    let (addr, addr_align) = if allow_higher_align && align > slot_size {
        (round_pointer_up_to_alignment(bx, ptr, align), align)
    } else {
        (ptr, slot_size)
    };

    let aligned_size = size.align_to(slot_size).bytes() as i32;
    let full_direct_size = bx.cx().const_i32(aligned_size);
    let next = bx.inbounds_ptradd(addr, full_direct_size);
    bx.store(next, va_list_addr, ptr_align_abi);

    if size.bytes() < slot_size.bytes()
        && bx.tcx().sess.target.endian == Endian::Big
        && force_right_adjust
    {
        let adjusted_size = bx.cx().const_i32((slot_size.bytes() - size.bytes()) as i32);
        let adjusted = bx.inbounds_ptradd(addr, adjusted_size);
        // We're in the middle of a slot now, so use the type's alignment, not the slot's.
        (adjusted, align)
    } else {
        (addr, addr_align)
    }
}

enum PassMode {
    Direct,
    Indirect,
}

enum SlotSize {
    Bytes8 = 8,
    Bytes4 = 4,
    Bytes1 = 1,
}

enum AllowHigherAlign {
    No,
    Yes,
}

enum ForceRightAdjust {
    No,
    Yes,
}

fn emit_ptr_va_arg<'ll, 'tcx>(
    bx: &mut Builder<'_, 'll, 'tcx>,
    list: OperandRef<'tcx, &'ll Value>,
    target_ty: Ty<'tcx>,
    pass_mode: PassMode,
    slot_size: SlotSize,
    allow_higher_align: AllowHigherAlign,
    force_right_adjust: ForceRightAdjust,
) -> &'ll Value {
    let indirect = matches!(pass_mode, PassMode::Indirect);
    let allow_higher_align = matches!(allow_higher_align, AllowHigherAlign::Yes);
    let force_right_adjust = matches!(force_right_adjust, ForceRightAdjust::Yes);
    let slot_size = Align::from_bytes(slot_size as u64).unwrap();

    let layout = bx.cx.layout_of(target_ty);
    let (llty, size, align) = if indirect {
        (
            bx.cx.layout_of(Ty::new_imm_ptr(bx.cx.tcx, target_ty)).llvm_type(bx.cx),
            bx.cx.data_layout().pointer_size(),
            bx.cx.data_layout().pointer_align(),
        )
    } else {
        (layout.llvm_type(bx.cx), layout.size, layout.align)
    };
    let (addr, addr_align) = emit_direct_ptr_va_arg(
        bx,
        list,
        size,
        align.abi,
        slot_size,
        allow_higher_align,
        force_right_adjust,
    );
    if indirect {
        let tmp_ret = bx.load(llty, addr, addr_align);
        bx.load(layout.llvm_type(bx.cx), tmp_ret, align.abi)
    } else {
        bx.load(llty, addr, addr_align)
    }
}

fn emit_aapcs_va_arg<'ll, 'tcx>(
    bx: &mut Builder<'_, 'll, 'tcx>,
    list: OperandRef<'tcx, &'ll Value>,
    target_ty: Ty<'tcx>,
) -> &'ll Value {
    let dl = bx.cx.data_layout();

    // Implementation of the AAPCS64 calling convention for va_args see
    // https://github.com/ARM-software/abi-aa/blob/master/aapcs64/aapcs64.rst
    //
    // typedef struct  va_list {
    //     void * stack; // next stack param
    //     void * gr_top; // end of GP arg reg save area
    //     void * vr_top; // end of FP/SIMD arg reg save area
    //     int gr_offs; // offset from  gr_top to next GP register arg
    //     int vr_offs; // offset from  vr_top to next FP/SIMD register arg
    // } va_list;
    let va_list_addr = list.immediate();

    // There is no padding between fields since `void*` is size=8 align=8, `int` is size=4 align=4.
    // See https://github.com/ARM-software/abi-aa/blob/master/aapcs64/aapcs64.rst
    // Table 1, Byte size and byte alignment of fundamental data types
    // Table 3, Mapping of C & C++ built-in data types
    let ptr_offset = 8;
    let i32_offset = 4;
    let gr_top = bx.inbounds_ptradd(va_list_addr, bx.cx.const_usize(ptr_offset));
    let vr_top = bx.inbounds_ptradd(va_list_addr, bx.cx.const_usize(2 * ptr_offset));
    let gr_offs = bx.inbounds_ptradd(va_list_addr, bx.cx.const_usize(3 * ptr_offset));
    let vr_offs = bx.inbounds_ptradd(va_list_addr, bx.cx.const_usize(3 * ptr_offset + i32_offset));

    let layout = bx.cx.layout_of(target_ty);

    let maybe_reg = bx.append_sibling_block("va_arg.maybe_reg");
    let in_reg = bx.append_sibling_block("va_arg.in_reg");
    let on_stack = bx.append_sibling_block("va_arg.on_stack");
    let end = bx.append_sibling_block("va_arg.end");
    let zero = bx.const_i32(0);
    let offset_align = Align::from_bytes(4).unwrap();

    let gr_type = target_ty.is_any_ptr() || target_ty.is_integral();
    let (reg_off, reg_top, slot_size) = if gr_type {
        let nreg = layout.size.bytes().div_ceil(8);
        (gr_offs, gr_top, nreg * 8)
    } else {
        let nreg = layout.size.bytes().div_ceil(16);
        (vr_offs, vr_top, nreg * 16)
    };

    // if the offset >= 0 then the value will be on the stack
    let mut reg_off_v = bx.load(bx.type_i32(), reg_off, offset_align);
    let use_stack = bx.icmp(IntPredicate::IntSGE, reg_off_v, zero);
    bx.cond_br(use_stack, on_stack, maybe_reg);

    // The value at this point might be in a register, but there is a chance that
    // it could be on the stack so we have to update the offset and then check
    // the offset again.

    bx.switch_to_block(maybe_reg);
    if gr_type && layout.align.bytes() > 8 {
        reg_off_v = bx.add(reg_off_v, bx.const_i32(15));
        reg_off_v = bx.and(reg_off_v, bx.const_i32(-16));
    }
    let new_reg_off_v = bx.add(reg_off_v, bx.const_i32(slot_size as i32));

    bx.store(new_reg_off_v, reg_off, offset_align);

    // Check to see if we have overflowed the registers as a result of this.
    // If we have then we need to use the stack for this value
    let use_stack = bx.icmp(IntPredicate::IntSGT, new_reg_off_v, zero);
    bx.cond_br(use_stack, on_stack, in_reg);

    bx.switch_to_block(in_reg);
    let top_type = bx.type_ptr();
    let top = bx.load(top_type, reg_top, dl.pointer_align().abi);

    // reg_value = *(@top + reg_off_v);
    let mut reg_addr = bx.ptradd(top, reg_off_v);
    if bx.tcx().sess.target.endian == Endian::Big && layout.size.bytes() != slot_size {
        // On big-endian systems the value is right-aligned in its slot.
        let offset = bx.const_i32((slot_size - layout.size.bytes()) as i32);
        reg_addr = bx.ptradd(reg_addr, offset);
    }
    let reg_type = layout.llvm_type(bx);
    let reg_value = bx.load(reg_type, reg_addr, layout.align.abi);
    bx.br(end);

    // On Stack block
    bx.switch_to_block(on_stack);
    let stack_value = emit_ptr_va_arg(
        bx,
        list,
        target_ty,
        PassMode::Direct,
        SlotSize::Bytes8,
        AllowHigherAlign::Yes,
        ForceRightAdjust::No,
    );
    bx.br(end);

    bx.switch_to_block(end);
    let val =
        bx.phi(layout.immediate_llvm_type(bx), &[reg_value, stack_value], &[in_reg, on_stack]);

    val
}

fn emit_powerpc_va_arg<'ll, 'tcx>(
    bx: &mut Builder<'_, 'll, 'tcx>,
    list: OperandRef<'tcx, &'ll Value>,
    target_ty: Ty<'tcx>,
) -> &'ll Value {
    let dl = bx.cx.data_layout();

    // struct __va_list_tag {
    //   unsigned char gpr;
    //   unsigned char fpr;
    //   unsigned short reserved;
    //   void *overflow_arg_area;
    //   void *reg_save_area;
    // };
    let va_list_addr = list.immediate();

    // Peel off any newtype wrappers.
    let layout = {
        let mut layout = bx.cx.layout_of(target_ty);

        while let Some((_, inner)) = layout.non_1zst_field(bx.cx) {
            layout = inner;
        }

        layout
    };

    // Rust does not currently support any powerpc softfloat targets.
    let target = &bx.cx.tcx.sess.target;
    let is_soft_float_abi = target.rustc_abi == Some(RustcAbi::Softfloat);
    assert!(!is_soft_float_abi);

    // All instances of VaArgSafe are passed directly.
    let is_indirect = false;

    let (is_i64, is_int, is_f64) = match layout.layout.backend_repr() {
        BackendRepr::Scalar(scalar) => match scalar.primitive() {
            rustc_abi::Primitive::Int(integer, _) => (integer.size().bits() == 64, true, false),
            rustc_abi::Primitive::Float(float) => (false, false, float.size().bits() == 64),
            rustc_abi::Primitive::Pointer(_) => (false, true, false),
        },
        _ => unreachable!("all instances of VaArgSafe are represented as scalars"),
    };

    let num_regs_addr = if is_int || is_soft_float_abi {
        va_list_addr // gpr
    } else {
        bx.inbounds_ptradd(va_list_addr, bx.const_usize(1)) // fpr
    };

    let mut num_regs = bx.load(bx.type_i8(), num_regs_addr, dl.i8_align);

    // "Align" the register count when the type is passed as `i64`.
    if is_i64 || (is_f64 && is_soft_float_abi) {
        num_regs = bx.add(num_regs, bx.const_u8(1));
        num_regs = bx.and(num_regs, bx.const_u8(0b1111_1110));
    }

    let max_regs = 8u8;
    let use_regs = bx.icmp(IntPredicate::IntULT, num_regs, bx.const_u8(max_regs));
    let ptr_align_abi = bx.tcx().data_layout.pointer_align().abi;

    let in_reg = bx.append_sibling_block("va_arg.in_reg");
    let in_mem = bx.append_sibling_block("va_arg.in_mem");
    let end = bx.append_sibling_block("va_arg.end");

    bx.cond_br(use_regs, in_reg, in_mem);

    let reg_addr = {
        bx.switch_to_block(in_reg);

        let reg_safe_area_ptr = bx.inbounds_ptradd(va_list_addr, bx.cx.const_usize(1 + 1 + 2 + 4));
        let mut reg_addr = bx.load(bx.type_ptr(), reg_safe_area_ptr, ptr_align_abi);

        // Floating-point registers start after the general-purpose registers.
        if !is_int && !is_soft_float_abi {
            reg_addr = bx.inbounds_ptradd(reg_addr, bx.cx.const_usize(32))
        }

        // Get the address of the saved value by scaling the number of
        // registers we've used by the number of.
        let reg_size = if is_int || is_soft_float_abi { 4 } else { 8 };
        let reg_offset = bx.mul(num_regs, bx.cx().const_u8(reg_size));
        let reg_addr = bx.inbounds_ptradd(reg_addr, reg_offset);

        // Increase the used-register count.
        let reg_incr = if is_i64 || (is_f64 && is_soft_float_abi) { 2 } else { 1 };
        let new_num_regs = bx.add(num_regs, bx.cx.const_u8(reg_incr));
        bx.store(new_num_regs, num_regs_addr, dl.i8_align);

        bx.br(end);

        reg_addr
    };

    let mem_addr = {
        bx.switch_to_block(in_mem);

        bx.store(bx.const_u8(max_regs), num_regs_addr, dl.i8_align);

        // Everything in the overflow area is rounded up to a size of at least 4.
        let overflow_area_align = Align::from_bytes(4).unwrap();

        let size = if !is_indirect {
            layout.layout.size.align_to(overflow_area_align)
        } else {
            dl.pointer_size()
        };

        let overflow_area_ptr = bx.inbounds_ptradd(va_list_addr, bx.cx.const_usize(1 + 1 + 2));
        let mut overflow_area = bx.load(bx.type_ptr(), overflow_area_ptr, ptr_align_abi);

        // Round up address of argument to alignment
        if layout.layout.align.abi > overflow_area_align {
            overflow_area =
                round_pointer_up_to_alignment(bx, overflow_area, layout.layout.align.abi);
        }

        let mem_addr = overflow_area;

        // Increase the overflow area.
        overflow_area = bx.inbounds_ptradd(overflow_area, bx.const_usize(size.bytes()));
        bx.store(overflow_area, overflow_area_ptr, ptr_align_abi);

        bx.br(end);

        mem_addr
    };

    // Return the appropriate result.
    bx.switch_to_block(end);
    let val_addr = bx.phi(bx.type_ptr(), &[reg_addr, mem_addr], &[in_reg, in_mem]);
    let val_type = layout.llvm_type(bx);
    let val_addr =
        if is_indirect { bx.load(bx.cx.type_ptr(), val_addr, ptr_align_abi) } else { val_addr };
    bx.load(val_type, val_addr, layout.align.abi)
}

fn emit_s390x_va_arg<'ll, 'tcx>(
    bx: &mut Builder<'_, 'll, 'tcx>,
    list: OperandRef<'tcx, &'ll Value>,
    target_ty: Ty<'tcx>,
) -> &'ll Value {
    let dl = bx.cx.data_layout();

    // Implementation of the s390x ELF ABI calling convention for va_args see
    // https://github.com/IBM/s390x-abi (chapter 1.2.4)
    //
    // typedef struct __va_list_tag {
    //     long __gpr;
    //     long __fpr;
    //     void *__overflow_arg_area;
    //     void *__reg_save_area;
    // } va_list[1];
    let va_list_addr = list.immediate();

    // There is no padding between fields since `long` and `void*` both have size=8 align=8.
    // https://github.com/IBM/s390x-abi (Table 1.1.: Scalar types)
    let i64_offset = 8;
    let ptr_offset = 8;
    let gpr = va_list_addr;
    let fpr = bx.inbounds_ptradd(va_list_addr, bx.cx.const_usize(i64_offset));
    let overflow_arg_area = bx.inbounds_ptradd(va_list_addr, bx.cx.const_usize(2 * i64_offset));
    let reg_save_area =
        bx.inbounds_ptradd(va_list_addr, bx.cx.const_usize(2 * i64_offset + ptr_offset));

    let layout = bx.cx.layout_of(target_ty);

    let in_reg = bx.append_sibling_block("va_arg.in_reg");
    let in_mem = bx.append_sibling_block("va_arg.in_mem");
    let end = bx.append_sibling_block("va_arg.end");
    let ptr_align_abi = dl.pointer_align().abi;

    // FIXME: vector ABI not yet supported.
    let target_ty_size = bx.cx.size_of(target_ty).bytes();
    let indirect: bool = target_ty_size > 8 || !target_ty_size.is_power_of_two();
    let unpadded_size = if indirect { 8 } else { target_ty_size };
    let padded_size = 8;
    let padding = padded_size - unpadded_size;

    let gpr_type = indirect || !layout.is_single_fp_element(bx.cx);
    let (max_regs, reg_count, reg_save_index, reg_padding) =
        if gpr_type { (5, gpr, 2, padding) } else { (4, fpr, 16, 0) };

    // Check whether the value was passed in a register or in memory.
    let reg_count_v = bx.load(bx.type_i64(), reg_count, Align::from_bytes(8).unwrap());
    let use_regs = bx.icmp(IntPredicate::IntULT, reg_count_v, bx.const_u64(max_regs));
    bx.cond_br(use_regs, in_reg, in_mem);

    // Emit code to load the value if it was passed in a register.
    bx.switch_to_block(in_reg);

    // Work out the address of the value in the register save area.
    let reg_ptr_v = bx.load(bx.type_ptr(), reg_save_area, ptr_align_abi);
    let scaled_reg_count = bx.mul(reg_count_v, bx.const_u64(8));
    let reg_off = bx.add(scaled_reg_count, bx.const_u64(reg_save_index * 8 + reg_padding));
    let reg_addr = bx.ptradd(reg_ptr_v, reg_off);

    // Update the register count.
    let new_reg_count_v = bx.add(reg_count_v, bx.const_u64(1));
    bx.store(new_reg_count_v, reg_count, Align::from_bytes(8).unwrap());
    bx.br(end);

    // Emit code to load the value if it was passed in memory.
    bx.switch_to_block(in_mem);

    // Work out the address of the value in the argument overflow area.
    let arg_ptr_v = bx.load(bx.type_ptr(), overflow_arg_area, ptr_align_abi);
    let arg_off = bx.const_u64(padding);
    let mem_addr = bx.ptradd(arg_ptr_v, arg_off);

    // Update the argument overflow area pointer.
    let arg_size = bx.cx().const_u64(padded_size);
    let new_arg_ptr_v = bx.inbounds_ptradd(arg_ptr_v, arg_size);
    bx.store(new_arg_ptr_v, overflow_arg_area, ptr_align_abi);
    bx.br(end);

    // Return the appropriate result.
    bx.switch_to_block(end);
    let val_addr = bx.phi(bx.type_ptr(), &[reg_addr, mem_addr], &[in_reg, in_mem]);
    let val_type = layout.llvm_type(bx);
    let val_addr =
        if indirect { bx.load(bx.cx.type_ptr(), val_addr, ptr_align_abi) } else { val_addr };
    bx.load(val_type, val_addr, layout.align.abi)
}

fn emit_x86_64_sysv64_va_arg<'ll, 'tcx>(
    bx: &mut Builder<'_, 'll, 'tcx>,
    list: OperandRef<'tcx, &'ll Value>,
    target_ty: Ty<'tcx>,
) -> &'ll Value {
    let dl = bx.cx.data_layout();

    // Implementation of the systemv x86_64 ABI calling convention for va_args, see
    // https://gitlab.com/x86-psABIs/x86-64-ABI (section 3.5.7). This implementation is heavily
    // based on the one in clang.

    // We're able to take some shortcuts because the return type of `va_arg` must implement the
    // `VaArgSafe` trait. Currently, only pointers, f64, i32, u32, i64 and u64 implement this trait.

    // typedef struct __va_list_tag {
    //     unsigned int gp_offset;
    //     unsigned int fp_offset;
    //     void *overflow_arg_area;
    //     void *reg_save_area;
    // } va_list[1];
    let va_list_addr = list.immediate();

    // Peel off any newtype wrappers.
    //
    // The "C" ABI does not unwrap newtypes (see `ReprOptions::inhibit_newtype_abi_optimization`).
    // Here, we do actually want the unwrapped representation, because that is how LLVM/Clang
    // pass such types to variadic functions.
    //
    // An example of a type that must be unwrapped is `Foo` below. Without the unwrapping, it has
    // `BackendRepr::Memory`, but we need it to be `BackendRepr::Scalar` to generate correct code.
    //
    // ```
    // #[repr(C)]
    // struct Empty;
    //
    // #[repr(C)]
    // struct Foo([Empty; 8], i32);
    // ```
    let layout = {
        let mut layout = bx.cx.layout_of(target_ty);

        while let Some((_, inner)) = layout.non_1zst_field(bx.cx) {
            layout = inner;
        }

        layout
    };

    // AMD64-ABI 3.5.7p5: Step 1. Determine whether type may be passed
    // in the registers. If not go to step 7.

    // AMD64-ABI 3.5.7p5: Step 2. Compute num_gp to hold the number of
    // general purpose registers needed to pass type and num_fp to hold
    // the number of floating point registers needed.

    let mut num_gp_registers = 0;
    let mut num_fp_registers = 0;

    let mut registers_for_primitive = |p| match p {
        Primitive::Int(integer, _is_signed) => {
            num_gp_registers += integer.size().bytes().div_ceil(8) as u32;
        }
        Primitive::Float(float) => {
            num_fp_registers += float.size().bytes().div_ceil(16) as u32;
        }
        Primitive::Pointer(_) => {
            num_gp_registers += 1;
        }
    };

    match layout.layout.backend_repr() {
        BackendRepr::Scalar(scalar) => {
            registers_for_primitive(scalar.primitive());
        }
        BackendRepr::ScalarPair(scalar1, scalar2) => {
            registers_for_primitive(scalar1.primitive());
            registers_for_primitive(scalar2.primitive());
        }
        BackendRepr::SimdVector { .. } | BackendRepr::SimdScalableVector { .. } => {
            // Because no instance of VaArgSafe uses a non-scalar `BackendRepr`.
            unreachable!(
                "No x86-64 SysV va_arg implementation for {:?}",
                layout.layout.backend_repr()
            )
        }
        BackendRepr::Memory { .. } => {
            let mem_addr = x86_64_sysv64_va_arg_from_memory(bx, va_list_addr, layout);
            return bx.load(layout.llvm_type(bx), mem_addr, layout.align.abi);
        }
    };

    // AMD64-ABI 3.5.7p5: Step 3. Verify whether arguments fit into
    // registers. In the case: l->gp_offset > 48 - num_gp * 8 or
    // l->fp_offset > 176 - num_fp * 16 go to step 7.

    let unsigned_int_offset = 4;
    let ptr_offset = 8;
    let gp_offset_ptr = va_list_addr;
    let fp_offset_ptr = bx.inbounds_ptradd(va_list_addr, bx.cx.const_usize(unsigned_int_offset));

    let gp_offset_v = bx.load(bx.type_i32(), gp_offset_ptr, Align::from_bytes(8).unwrap());
    let fp_offset_v = bx.load(bx.type_i32(), fp_offset_ptr, Align::from_bytes(4).unwrap());

    let mut use_regs = bx.const_bool(false);

    if num_gp_registers > 0 {
        let max_offset_val = 48u32 - num_gp_registers * 8;
        let fits_in_gp = bx.icmp(IntPredicate::IntULE, gp_offset_v, bx.const_u32(max_offset_val));
        use_regs = fits_in_gp;
    }

    if num_fp_registers > 0 {
        let max_offset_val = 176u32 - num_fp_registers * 16;
        let fits_in_fp = bx.icmp(IntPredicate::IntULE, fp_offset_v, bx.const_u32(max_offset_val));
        use_regs = if num_gp_registers > 0 { bx.and(use_regs, fits_in_fp) } else { fits_in_fp };
    }

    let in_reg = bx.append_sibling_block("va_arg.in_reg");
    let in_mem = bx.append_sibling_block("va_arg.in_mem");
    let end = bx.append_sibling_block("va_arg.end");

    bx.cond_br(use_regs, in_reg, in_mem);

    // Emit code to load the value if it was passed in a register.
    bx.switch_to_block(in_reg);

    // AMD64-ABI 3.5.7p5: Step 4. Fetch type from l->reg_save_area with
    // an offset of l->gp_offset and/or l->fp_offset. This may require
    // copying to a temporary location in case the parameter is passed
    // in different register classes or requires an alignment greater
    // than 8 for general purpose registers and 16 for XMM registers.
    //
    // FIXME(llvm): This really results in shameful code when we end up needing to
    // collect arguments from different places; often what should result in a
    // simple assembling of a structure from scattered addresses has many more
    // loads than necessary. Can we clean this up?
    let reg_save_area_ptr =
        bx.inbounds_ptradd(va_list_addr, bx.cx.const_usize(2 * unsigned_int_offset + ptr_offset));
    let reg_save_area_v = bx.load(bx.type_ptr(), reg_save_area_ptr, dl.pointer_align().abi);

    let reg_addr = match layout.layout.backend_repr() {
        BackendRepr::Scalar(scalar) => match scalar.primitive() {
            Primitive::Int(_, _) | Primitive::Pointer(_) => {
                let reg_addr = bx.inbounds_ptradd(reg_save_area_v, gp_offset_v);

                // Copy into a temporary if the type is more aligned than the register save area.
                let gp_align = Align::from_bytes(8).unwrap();
                copy_to_temporary_if_more_aligned(bx, reg_addr, layout, gp_align)
            }
            Primitive::Float(_) => bx.inbounds_ptradd(reg_save_area_v, fp_offset_v),
        },
        BackendRepr::ScalarPair(scalar1, scalar2) => {
            let ty_lo = bx.cx().scalar_pair_element_backend_type(layout, 0, false);
            let ty_hi = bx.cx().scalar_pair_element_backend_type(layout, 1, false);

            let align_lo = layout.field(bx.cx, 0).layout.align().abi;
            let align_hi = layout.field(bx.cx, 1).layout.align().abi;

            match (scalar1.primitive(), scalar2.primitive()) {
                (Primitive::Float(_), Primitive::Float(_)) => {
                    // SSE registers are spaced 16 bytes apart in the register save
                    // area, we need to collect the two eightbytes together.
                    // The ABI isn't explicit about this, but it seems reasonable
                    // to assume that the slots are 16-byte aligned, since the stack is
                    // naturally 16-byte aligned and the prologue is expected to store
                    // all the SSE registers to the RSA.
                    let reg_lo_addr = bx.inbounds_ptradd(reg_save_area_v, fp_offset_v);
                    let reg_hi_addr = bx.inbounds_ptradd(reg_lo_addr, bx.const_i32(16));

                    let align = layout.layout.align().abi;
                    let tmp = bx.alloca(layout.layout.size(), align);

                    let reg_lo = bx.load(ty_lo, reg_lo_addr, align_lo);
                    let reg_hi = bx.load(ty_hi, reg_hi_addr, align_hi);

                    let offset = scalar1.size(bx.cx).align_to(align_hi).bytes();
                    let field0 = tmp;
                    let field1 = bx.inbounds_ptradd(tmp, bx.const_u32(offset as u32));

                    bx.store(reg_lo, field0, align);
                    bx.store(reg_hi, field1, align);

                    tmp
                }
                (Primitive::Float(_), _) | (_, Primitive::Float(_)) => {
                    let gp_addr = bx.inbounds_ptradd(reg_save_area_v, gp_offset_v);
                    let fp_addr = bx.inbounds_ptradd(reg_save_area_v, fp_offset_v);

                    let (reg_lo_addr, reg_hi_addr) = match scalar1.primitive() {
                        Primitive::Float(_) => (fp_addr, gp_addr),
                        Primitive::Int(_, _) | Primitive::Pointer(_) => (gp_addr, fp_addr),
                    };

                    let tmp = bx.alloca(layout.layout.size(), layout.layout.align().abi);

                    let reg_lo = bx.load(ty_lo, reg_lo_addr, align_lo);
                    let reg_hi = bx.load(ty_hi, reg_hi_addr, align_hi);

                    let offset = scalar1.size(bx.cx).align_to(align_hi).bytes();
                    let field0 = tmp;
                    let field1 = bx.inbounds_ptradd(tmp, bx.const_u32(offset as u32));

                    bx.store(reg_lo, field0, align_lo);
                    bx.store(reg_hi, field1, align_hi);

                    tmp
                }
                (_, _) => {
                    // Two integer/pointer values are just contiguous in memory.
                    let reg_addr = bx.inbounds_ptradd(reg_save_area_v, gp_offset_v);

                    // Copy into a temporary if the type is more aligned than the register save area.
                    let gp_align = Align::from_bytes(8).unwrap();
                    copy_to_temporary_if_more_aligned(bx, reg_addr, layout, gp_align)
                }
            }
        }
        // The Previous match on `BackendRepr` means control flow already escaped.
        BackendRepr::SimdVector { .. }
        | BackendRepr::SimdScalableVector { .. }
        | BackendRepr::Memory { .. } => unreachable!(),
    };

    // AMD64-ABI 3.5.7p5: Step 5. Set:
    // l->gp_offset = l->gp_offset + num_gp * 8
    if num_gp_registers > 0 {
        let offset = bx.const_u32(num_gp_registers * 8);
        let sum = bx.add(gp_offset_v, offset);
        // An alignment of 8 because `__va_list_tag` is 8-aligned and this is its first field.
        bx.store(sum, gp_offset_ptr, Align::from_bytes(8).unwrap());
    }

    // l->fp_offset = l->fp_offset + num_fp * 16.
    if num_fp_registers > 0 {
        let offset = bx.const_u32(num_fp_registers * 16);
        let sum = bx.add(fp_offset_v, offset);
        bx.store(sum, fp_offset_ptr, Align::from_bytes(4).unwrap());
    }

    bx.br(end);

    bx.switch_to_block(in_mem);
    let mem_addr = x86_64_sysv64_va_arg_from_memory(bx, va_list_addr, layout);
    bx.br(end);

    bx.switch_to_block(end);

    let val_type = layout.llvm_type(bx);
    let val_addr = bx.phi(bx.type_ptr(), &[reg_addr, mem_addr], &[in_reg, in_mem]);

    bx.load(val_type, val_addr, layout.align.abi)
}

/// Copy into a temporary if the type is more aligned than the register save area.
fn copy_to_temporary_if_more_aligned<'ll, 'tcx>(
    bx: &mut Builder<'_, 'll, 'tcx>,
    reg_addr: &'ll Value,
    layout: TyAndLayout<'tcx>,
    src_align: Align,
) -> &'ll Value {
    if layout.layout.align.abi > src_align {
        let tmp = bx.alloca(layout.layout.size(), layout.layout.align().abi);
        bx.memcpy(
            tmp,
            layout.layout.align.abi,
            reg_addr,
            src_align,
            bx.const_u32(layout.layout.size().bytes() as u32),
            MemFlags::empty(),
            None,
        );
        tmp
    } else {
        reg_addr
    }
}

fn x86_64_sysv64_va_arg_from_memory<'ll, 'tcx>(
    bx: &mut Builder<'_, 'll, 'tcx>,
    va_list_addr: &'ll Value,
    layout: TyAndLayout<'tcx>,
) -> &'ll Value {
    let dl = bx.cx.data_layout();
    let ptr_align_abi = dl.data_layout().pointer_align().abi;

    let overflow_arg_area_ptr = bx.inbounds_ptradd(va_list_addr, bx.const_usize(8));

    let overflow_arg_area_v = bx.load(bx.type_ptr(), overflow_arg_area_ptr, ptr_align_abi);
    // AMD64-ABI 3.5.7p5: Step 7. Align l->overflow_arg_area upwards to a 16
    // byte boundary if alignment needed by type exceeds 8 byte boundary.
    // It isn't stated explicitly in the standard, but in practice we use
    // alignment greater than 16 where necessary.
    if layout.layout.align.bytes() > 8 {
        unreachable!("all instances of VaArgSafe have an alignment <= 8");
    }

    // AMD64-ABI 3.5.7p5: Step 8. Fetch type from l->overflow_arg_area.
    let mem_addr = overflow_arg_area_v;

    // AMD64-ABI 3.5.7p5: Step 9. Set l->overflow_arg_area to:
    // l->overflow_arg_area + sizeof(type).
    // AMD64-ABI 3.5.7p5: Step 10. Align l->overflow_arg_area upwards to
    // an 8 byte boundary.
    let size_in_bytes = layout.layout.size().bytes();
    let offset = bx.const_i32(size_in_bytes.next_multiple_of(8) as i32);
    let overflow_arg_area = bx.inbounds_ptradd(overflow_arg_area_v, offset);
    bx.store(overflow_arg_area, overflow_arg_area_ptr, ptr_align_abi);

    mem_addr
}

fn emit_hexagon_va_arg_musl<'ll, 'tcx>(
    bx: &mut Builder<'_, 'll, 'tcx>,
    list: OperandRef<'tcx, &'ll Value>,
    target_ty: Ty<'tcx>,
) -> &'ll Value {
    // Implementation of va_arg for Hexagon musl target.
    // Based on LLVM's HexagonBuiltinVaList implementation.
    //
    // struct __va_list_tag {
    //   void *__current_saved_reg_area_pointer;
    //   void *__saved_reg_area_end_pointer;
    //   void *__overflow_area_pointer;
    // };
    //
    // All variadic arguments are passed on the stack, but the musl implementation
    //  uses a register save area for compatibility.
    let va_list_addr = list.immediate();
    let layout = bx.cx.layout_of(target_ty);
    let ptr_align_abi = bx.tcx().data_layout.pointer_align().abi;
    let ptr_size = bx.tcx().data_layout.pointer_size().bytes();

    // Check if argument fits in register save area
    let maybe_reg = bx.append_sibling_block("va_arg.maybe_reg");
    let from_overflow = bx.append_sibling_block("va_arg.from_overflow");
    let end = bx.append_sibling_block("va_arg.end");

    // Load the three pointers from va_list
    let current_ptr_addr = va_list_addr;
    let end_ptr_addr = bx.inbounds_ptradd(va_list_addr, bx.const_usize(ptr_size));
    let overflow_ptr_addr = bx.inbounds_ptradd(va_list_addr, bx.const_usize(2 * ptr_size));

    let current_ptr = bx.load(bx.type_ptr(), current_ptr_addr, ptr_align_abi);
    let end_ptr = bx.load(bx.type_ptr(), end_ptr_addr, ptr_align_abi);
    let overflow_ptr = bx.load(bx.type_ptr(), overflow_ptr_addr, ptr_align_abi);

    // Align current pointer based on argument type size (following LLVM's implementation)
    // Arguments <= 32 bits (4 bytes) use 4-byte alignment, > 32 bits use 8-byte alignment
    let type_size_bits = bx.cx.size_of(target_ty).bits();
    let arg_align = if type_size_bits > 32 {
        Align::from_bytes(8).unwrap()
    } else {
        Align::from_bytes(4).unwrap()
    };
    let aligned_current = round_pointer_up_to_alignment(bx, current_ptr, arg_align);

    // Calculate next pointer position (following LLVM's logic)
    // Arguments <= 32 bits take 4 bytes, > 32 bits take 8 bytes
    let arg_size = if type_size_bits > 32 { 8 } else { 4 };
    let next_ptr = bx.inbounds_ptradd(aligned_current, bx.const_usize(arg_size));

    // Check if argument fits in register save area
    let fits_in_regs = bx.icmp(IntPredicate::IntULE, next_ptr, end_ptr);
    bx.cond_br(fits_in_regs, maybe_reg, from_overflow);

    // Load from register save area
    bx.switch_to_block(maybe_reg);
    let reg_value_addr = aligned_current;
    // Update current pointer
    bx.store(next_ptr, current_ptr_addr, ptr_align_abi);
    bx.br(end);

    // Load from overflow area (stack)
    bx.switch_to_block(from_overflow);

    // Align overflow pointer using the same alignment rules
    let aligned_overflow = round_pointer_up_to_alignment(bx, overflow_ptr, arg_align);

    let overflow_value_addr = aligned_overflow;
    // Update overflow pointer - use the same size calculation
    let next_overflow = bx.inbounds_ptradd(aligned_overflow, bx.const_usize(arg_size));
    bx.store(next_overflow, overflow_ptr_addr, ptr_align_abi);

    // IMPORTANT: Also update the current saved register area pointer to match
    // This synchronizes the pointers when switching to overflow area
    bx.store(next_overflow, current_ptr_addr, ptr_align_abi);
    bx.br(end);

    // Return the value
    bx.switch_to_block(end);
    let value_addr =
        bx.phi(bx.type_ptr(), &[reg_value_addr, overflow_value_addr], &[maybe_reg, from_overflow]);
    bx.load(layout.llvm_type(bx), value_addr, layout.align.abi)
}

fn emit_hexagon_va_arg_bare_metal<'ll, 'tcx>(
    bx: &mut Builder<'_, 'll, 'tcx>,
    list: OperandRef<'tcx, &'ll Value>,
    target_ty: Ty<'tcx>,
) -> &'ll Value {
    // Implementation of va_arg for Hexagon bare-metal (non-musl) targets.
    // Based on LLVM's EmitVAArgForHexagon implementation.
    //
    // va_list is a simple pointer (char *)
    let va_list_addr = list.immediate();
    let layout = bx.cx.layout_of(target_ty);
    let ptr_align_abi = bx.tcx().data_layout.pointer_align().abi;

    // Load current pointer from va_list
    let current_ptr = bx.load(bx.type_ptr(), va_list_addr, ptr_align_abi);

    // Handle address alignment for types with alignment > 4 bytes
    let ty_align = layout.align.abi;
    let aligned_ptr = if ty_align.bytes() > 4 {
        // Ensure alignment is a power of 2
        debug_assert!(ty_align.bytes().is_power_of_two(), "Alignment is not power of 2!");
        round_pointer_up_to_alignment(bx, current_ptr, ty_align)
    } else {
        current_ptr
    };

    // Calculate offset: round up type size to 4-byte boundary (minimum stack slot size)
    let type_size = layout.size.bytes();
    let offset = type_size.next_multiple_of(4); // align to 4 bytes

    // Update va_list to point to next argument
    let next_ptr = bx.inbounds_ptradd(aligned_ptr, bx.const_usize(offset));
    bx.store(next_ptr, va_list_addr, ptr_align_abi);

    // Load and return the argument value
    bx.load(layout.llvm_type(bx), aligned_ptr, layout.align.abi)
}

fn emit_xtensa_va_arg<'ll, 'tcx>(
    bx: &mut Builder<'_, 'll, 'tcx>,
    list: OperandRef<'tcx, &'ll Value>,
    target_ty: Ty<'tcx>,
) -> &'ll Value {
    // Implementation of va_arg for Xtensa. There doesn't seem to be an authoritative source for
    // this, other than "what GCC does".
    //
    // The va_list type has three fields:
    // struct __va_list_tag {
    //   int32_t *va_stk; // Arguments passed on the stack
    //   int32_t *va_reg; // Arguments passed in registers, saved to memory by the prologue.
    //   int32_t va_ndx; // Offset into the arguments, in bytes
    // };
    //
    // The first 24 bytes (equivalent to 6 registers) come from va_reg, the rest from va_stk.
    // Thus if va_ndx is less than 24, the next va_arg *may* read from va_reg,
    // otherwise it must come from va_stk.
    //
    // Primitive arguments are never split between registers and the stack. For example, if loading an 8 byte
    // primitive value and va_ndx = 20, we instead bump the offset and read everything from va_stk.
    let va_list_addr = list.immediate();
    // FIXME: handle multi-field structs that split across regsave/stack?
    let layout = bx.cx.layout_of(target_ty);
    let from_stack = bx.append_sibling_block("va_arg.from_stack");
    let from_regsave = bx.append_sibling_block("va_arg.from_regsave");
    let end = bx.append_sibling_block("va_arg.end");
    let ptr_align_abi = bx.tcx().data_layout.pointer_align().abi;

    // (*va).va_ndx
    let va_reg_offset = 4;
    let va_ndx_offset = va_reg_offset + 4;
    let offset_ptr = bx.inbounds_ptradd(va_list_addr, bx.cx.const_usize(va_ndx_offset));

    let offset = bx.load(bx.type_i32(), offset_ptr, bx.tcx().data_layout.i32_align);
    let offset = round_up_to_alignment(bx, offset, layout.align.abi);

    let slot_size = layout.size.align_to(Align::from_bytes(4).unwrap()).bytes() as i32;

    // Update the offset in va_list, by adding the slot's size.
    let offset_next = bx.add(offset, bx.const_i32(slot_size));

    // Figure out where to look for our value. We do that by checking the end of our slot (offset_next).
    // If that is within the regsave area, then load from there. Otherwise load from the stack area.
    let regsave_size = bx.const_i32(24);
    let use_regsave = bx.icmp(IntPredicate::IntULE, offset_next, regsave_size);
    bx.cond_br(use_regsave, from_regsave, from_stack);

    bx.switch_to_block(from_regsave);
    // update va_ndx
    bx.store(offset_next, offset_ptr, ptr_align_abi);

    // (*va).va_reg
    let regsave_area_ptr = bx.inbounds_ptradd(va_list_addr, bx.cx.const_usize(va_reg_offset));
    let regsave_area = bx.load(bx.type_ptr(), regsave_area_ptr, ptr_align_abi);
    let regsave_value_ptr = bx.inbounds_ptradd(regsave_area, offset);
    bx.br(end);

    bx.switch_to_block(from_stack);

    // The first time we switch from regsave to stack we needs to adjust our offsets a bit.
    // va_stk is set up such that the first stack argument is always at va_stk + 32.
    // The corrected offset is written back into the va_list struct.

    // let offset_corrected = cmp::max(offset, 32);
    let stack_offset_start = bx.const_i32(32);
    let needs_correction = bx.icmp(IntPredicate::IntULE, offset, stack_offset_start);
    let offset_corrected = bx.select(needs_correction, stack_offset_start, offset);

    // let offset_next_corrected = offset_corrected + slot_size;
    // va_ndx = offset_next_corrected;
    let offset_next_corrected = bx.add(offset_next, bx.const_i32(slot_size));
    // update va_ndx
    bx.store(offset_next_corrected, offset_ptr, ptr_align_abi);

    // let stack_value_ptr = unsafe { (*va).va_stk.byte_add(offset_corrected) };
    let stack_area_ptr = bx.inbounds_ptradd(va_list_addr, bx.cx.const_usize(0));
    let stack_area = bx.load(bx.type_ptr(), stack_area_ptr, ptr_align_abi);
    let stack_value_ptr = bx.inbounds_ptradd(stack_area, offset_corrected);
    bx.br(end);

    bx.switch_to_block(end);

    // On big-endian, for values smaller than the slot size we'd have to align the read to the end
    // of the slot rather than the start. While the ISA and GCC support big-endian, all the Xtensa
    // targets supported by rustc are little-endian so don't worry about it.

    // if from_regsave {
    //     unsafe { *regsave_value_ptr }
    // } else {
    //     unsafe { *stack_value_ptr }
    // }
    assert!(bx.tcx().sess.target.endian == Endian::Little);
    let value_ptr =
        bx.phi(bx.type_ptr(), &[regsave_value_ptr, stack_value_ptr], &[from_regsave, from_stack]);
    return bx.load(layout.llvm_type(bx), value_ptr, layout.align.abi);
}

/// Determine the va_arg implementation to use. The LLVM va_arg instruction
/// is lacking in some instances, so we should only use it as a fallback.
///
/// <https://llvm.org/docs/LangRef.html#va-arg-instruction>
pub(super) fn emit_va_arg<'ll, 'tcx>(
    bx: &mut Builder<'_, 'll, 'tcx>,
    addr: OperandRef<'tcx, &'ll Value>,
    target_ty: Ty<'tcx>,
) -> &'ll Value {
    let layout = bx.cx.layout_of(target_ty);
    let target_ty_size = layout.layout.size().bytes();

    // Some ABIs have special behavior for zero-sized types. currently `VaArgSafe` is not
    // implemented for any zero-sized types, so this assert should always hold.
    assert!(!bx.layout_of(target_ty).is_zst());

    let target = &bx.cx.tcx.sess.target;
    match target.arch {
        Arch::X86 => emit_ptr_va_arg(
            bx,
            addr,
            target_ty,
            PassMode::Direct,
            SlotSize::Bytes4,
            if target.is_like_windows { AllowHigherAlign::No } else { AllowHigherAlign::Yes },
            ForceRightAdjust::No,
        ),
        Arch::Arm64EC => emit_ptr_va_arg(
            bx,
            addr,
            target_ty,
            PassMode::Direct,
            SlotSize::Bytes8,
            if target.is_like_windows { AllowHigherAlign::No } else { AllowHigherAlign::Yes },
            ForceRightAdjust::No,
        ),
        Arch::AArch64 if target.is_like_windows || target.is_like_darwin => emit_ptr_va_arg(
            bx,
            addr,
            target_ty,
            PassMode::Direct,
            SlotSize::Bytes8,
            if target.is_like_windows { AllowHigherAlign::No } else { AllowHigherAlign::Yes },
            ForceRightAdjust::No,
        ),
        Arch::AArch64 => emit_aapcs_va_arg(bx, addr, target_ty),
        Arch::Arm => {
            // Types wider than 16 bytes are not currently supported. Clang has special logic for
            // such types, but `VaArgSafe` is not implemented for any type that is this large.
            assert!(bx.cx.size_of(target_ty).bytes() <= 16);

            emit_ptr_va_arg(
                bx,
                addr,
                target_ty,
                PassMode::Direct,
                SlotSize::Bytes4,
                AllowHigherAlign::Yes,
                ForceRightAdjust::No,
            )
        }
        Arch::S390x => emit_s390x_va_arg(bx, addr, target_ty),
        Arch::PowerPC => emit_powerpc_va_arg(bx, addr, target_ty),
        Arch::PowerPC64 => emit_ptr_va_arg(
            bx,
            addr,
            target_ty,
            PassMode::Direct,
            SlotSize::Bytes8,
            AllowHigherAlign::Yes,
            ForceRightAdjust::Yes,
        ),
        Arch::RiscV32 if target.llvm_abiname == LlvmAbi::Ilp32e => {
            // FIXME: clang manually adjusts the alignment for this ABI. It notes:
            //
            // > To be compatible with GCC's behaviors, we force arguments with
            // > 2×XLEN-bit alignment and size at most 2×XLEN bits like `long long`,
            // > `unsigned long long` and `double` to have 4-byte alignment. This
            // > behavior may be changed when RV32E/ILP32E is ratified.
            bug!("c-variadic calls with ilp32e use a custom ABI and are not currently implemented");
        }
        Arch::RiscV32 | Arch::LoongArch32 => emit_ptr_va_arg(
            bx,
            addr,
            target_ty,
            if target_ty_size > 2 * 4 { PassMode::Indirect } else { PassMode::Direct },
            SlotSize::Bytes4,
            AllowHigherAlign::Yes,
            ForceRightAdjust::No,
        ),
        Arch::RiscV64 | Arch::LoongArch64 => emit_ptr_va_arg(
            bx,
            addr,
            target_ty,
            if target_ty_size > 2 * 8 { PassMode::Indirect } else { PassMode::Direct },
            SlotSize::Bytes8,
            AllowHigherAlign::Yes,
            ForceRightAdjust::No,
        ),
        Arch::AmdGpu => emit_ptr_va_arg(
            bx,
            addr,
            target_ty,
            PassMode::Direct,
            SlotSize::Bytes4,
            AllowHigherAlign::No,
            ForceRightAdjust::No,
        ),
        Arch::Nvptx64 => emit_ptr_va_arg(
            bx,
            addr,
            target_ty,
            PassMode::Direct,
            SlotSize::Bytes1,
            AllowHigherAlign::Yes,
            ForceRightAdjust::No,
        ),
        Arch::Wasm32 | Arch::Wasm64 => emit_ptr_va_arg(
            bx,
            addr,
            target_ty,
            if layout.is_aggregate() || layout.is_zst() || layout.is_1zst() {
                PassMode::Indirect
            } else {
                PassMode::Direct
            },
            SlotSize::Bytes4,
            AllowHigherAlign::Yes,
            ForceRightAdjust::No,
        ),
        Arch::CSky => emit_ptr_va_arg(
            bx,
            addr,
            target_ty,
            PassMode::Direct,
            SlotSize::Bytes4,
            AllowHigherAlign::Yes,
            ForceRightAdjust::No,
        ),
        // Windows x86_64
        Arch::X86_64 if target.is_like_windows => emit_ptr_va_arg(
            bx,
            addr,
            target_ty,
            if target_ty_size > 8 || !target_ty_size.is_power_of_two() {
                PassMode::Indirect
            } else {
                PassMode::Direct
            },
            SlotSize::Bytes8,
            AllowHigherAlign::No,
            ForceRightAdjust::No,
        ),
        // This includes `target.is_like_darwin`, which on x86_64 targets is like sysv64.
        Arch::X86_64 => emit_x86_64_sysv64_va_arg(bx, addr, target_ty),
        Arch::Xtensa => emit_xtensa_va_arg(bx, addr, target_ty),
        Arch::Hexagon => match target.env {
            Env::Musl => emit_hexagon_va_arg_musl(bx, addr, target_ty),
            _ => emit_hexagon_va_arg_bare_metal(bx, addr, target_ty),
        },
        Arch::Sparc64 => emit_ptr_va_arg(
            bx,
            addr,
            target_ty,
            if target_ty_size > 2 * 8 { PassMode::Indirect } else { PassMode::Direct },
            SlotSize::Bytes8,
            AllowHigherAlign::Yes,
            ForceRightAdjust::No,
        ),

        Arch::Bpf => bug!("bpf does not support c-variadic functions"),
        Arch::SpirV => bug!("spirv does not support c-variadic functions"),

        Arch::Mips | Arch::Mips32r6 | Arch::Mips64 | Arch::Mips64r6 => {
            // FIXME: port MipsTargetLowering::lowerVAARG.
            bx.va_arg(addr.immediate(), bx.cx.layout_of(target_ty).llvm_type(bx.cx))
        }
        Arch::Sparc | Arch::Avr | Arch::M68k | Arch::Msp430 => {
            // Clang uses the LLVM implementation for these architectures.
            bx.va_arg(addr.immediate(), bx.cx.layout_of(target_ty).llvm_type(bx.cx))
        }
        Arch::Other(ref arch) => {
            // Just to be safe we error out explicitly here, instead of crossing our fingers that
            // the default LLVM implementation has the correct behavior for this target.
            bug!("c-variadic functions are not currently implemented for custom target {arch}")
        }
    }
}